Gas is trapped when it has nowhere to go, i.e., one-side-tented vias. You tent both sides, it's fine, it has somewhere to go. That's why you use untented, or fully tented (preferably plugged and capped) vias, for via-in-pad.
Buoyancy probably doesn't make much difference, as surface tension dominates on these scales. Maybe it would help some, maybe it would reduce but not eliminate the rate of voiding, for example. Dunno, would have to see it tested. Meanwhile, parts simply bubbling/popping off the inverted board (or get shaken off during handling), is probably the elephant in the room.
Tim